Preparation of 2,4-dihydroxybenzoic acid

The present invention relates to a novel process for preparing 2,4-dihydroxybenzoic acid in a solids mixer by reacting resorcinol with an alkali metal bicarbonate or carbonate in a carbon dioxide atmosphere, which comprises using as the alkali metal bicarbonate or carbonate a mixture of the corresponding sodium and potassium salts.

The preparation of benzoic acids by carboxylation in an alkali medium is known as a Kolbe synthesis. For resorcinol as the aromatic species the reaction in a solvent, which in general gives a poor yield and low selectivity, is not the only option because it is also possible to perform a Marasse solid-phase carboxylation with carbon dioxide in the presence of potassium carbonate (J. Org. Chem. 19, (1954) 510; Monatsh. 81, (1950) 1071; J. Org. Chem. 15, (1950), 233).

It is generally held that the Marasse reaction can only be carried out successfully using potassium, rubidium or cesium salts. The less costly sodium salts, by contrast, are held to be unsuitable (Chem. Rev. 57, (1957), 583).

It is an object of the present invention to provide a process for preparing 2,4-dihydroxybenzoic acid by solid-phase carboxylation using sodium bicarbonate or carbonate.

We have found that this object is achieved by preparing 2,4-dihydroxybenzoic acid in a solids mixer by reacting resorcinol with an alkali metal bicarbonate, alkali metal carbonate or a mixture thereof in a carbon dioxide atmosphere if the alkali metal bicarbonate or carbonate used is a mixture of the corresponding sodium and potassium salts.

The proportion of potassium salts in the mixture is in general from 10 to 60% by weight, preferably from 20 to 40% by weight, each percentage being based on the weight of the total of sodium and potassium salt.

Preference is given to using a mixture of sodium bicarbonate and potassium bicarbonate.

In principle it is also possible to carry out the reaction in the presence of sodium bicarbonate and/or sodium carbonate alone, but in that case a longer reaction time is required.

In general, from 400 to 1500 g, preferably from 450 to 700 g, in particular from 500 to 600 g, of the mixture of sodium bicarbonate and potassium bicarbonate and/or carbonate are used per mole of resorcinol.

The process according to the invention is advantageously carried out at from 80 to 140° C., preferably at from 100 to 120° C.

The carbon dioxide required for the reaction is preferably supplied to the reaction under atmospheric pressure. However, it is also possible to work under superatmospheric pressure, in general at up to about 20 bar. The carbon dioxide can also be diluted by inert gases, for example nitrogen, up to a pressure of 0.5 bar.

The reaction mixture may further contain inert substances, for example inorganic salts, for example halides, sulfates or phosphates of alkali metals or alkaline earth metals or even alkaline earth metal bicarbonates or alkaline earth metal carbonates, without adversely affecting the reaction. Furthermore, it is also possible to add inert materials such as minerals or metals. In general, the amount of inert substances added is from 0 to 200% by weight, preferably from 0 to 100% by weight, based on the reaction mixture.

The process according to the invention is carried out in a solids mixer. Such apparatus is known and described for example in Ullmanns Encyklopadie der Technischen Chemie, 4th edition, volume 2, pages 304-310.

The novel process is advantageously carried out by initially charging the solids mixer with the resorcinol and the mixture of sodium bicarbonate and potassium bicarbonate and/or carbonate with or without any of the abovementioned inert substances, and adding carbon dioxide under mixing conditions. Following heating to the abovementioned temperature and a reaction time of about 1 to 10 hours, preferably about 2 to 4 hours, under the abovementioned reaction conditions, the reaction will have ended.

To work up the solid reaction mixture, it can be introduced in water and the resulting mixture be acidified. The 2,4-dihydroxybenzoic acid which precipitates can then be separated off and dried.

It is also possible to extract the solid reaction mixture with lower alcohols, such as methanol, ethanol or isopropanol, at an extraction temperature of from room temperature to the boiling point of the particular extractant used. After the extractant has been removed, the result obtained is a mixture of the sodium and potassium salt of 2,4-dihydroxybenzoic acid, which may either be used as such or be converted into the free acid by acidification. The extraction residue largely comprises sodium bicarbonate and potassium bicarbonate and/or carbonate and, after drying, may be used again in the solid-phase reaction.

It is surprising that the process according to the invention, which may be carried out not only continuously but also batchwise, permits the preparation of 2,4-dihydroxybenzoic acid in the presence of sodium bicarbonate and/or carbonate although this was considered impossible by the prior art.

It is also surprising that the mixture of the sodium and potassium salts of 2,4-dihydroxybenzoic acid can be extracted from the reaction mixture by means of lower alcohols.

2,4-Dihydroxybenzoic acid is a useful intermediate for preparing UV absorbers.

The following Examples will illustrate the invention in more detail: